Bleaching procedure using chlorine dioxide and chlorine solutions

ABSTRACT

The adverse effects of dissolved organic material in the aqueous phase of pulp suspensions during bleaching using mixtures of chlorine dioxide and chlorine are overcome by applying the chlorine dioxide and chlorine serially to the pulp in two steps without an intermediate wash. An aqueous solution of chlorine dioxide and chlorine in which the chlorine constitutes from about 6 to about 10% of the total available chlorine of the solution is used in the first step and chlorine is used in the second step. The aqueous solutions are formed by dissolving in water chlorine dioxide and chlorine formed in a chlorine dioxide generator.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 098,190 filed Nov. 22, 1979 (now U.S. Pat. No.4,325,783) which itself is a continuation-in-part of U.S. patentapplication Ser. No. 023,845, filed Mar. 26, 1979, now abandoned.

FIELD OF INVENTION

The present invention relates to the bleaching of cellulosic fibrousmaterial, more particularly to the bleaching of wood pulp.

BACKGROUND TO THE INVENTION

In multistage pulp bleaching and purification processes wherein woodpulp is subjected to a plurality of bleaching and caustic extractionstages, typically using mixtures of chlorine dioxide and chlorine in thefirst bleaching stage, chlorine dioxide in subsequent bleaching stagesand sodium hydroxide in the caustic extraction stages, and the pulp iswashed intermediate each such bleaching and caustic extraction stage,the volume of effluents from the multistage bleaching process and theoverall fresh water requirement are decreased by effectingcountercurrent washing operations within the multistage process and byusing effluents from the bleach plant to wash the unbleached pulp duringto its passage from the digester to the bleach plant, preferablyutilizing the procedure described in U.S. Pat. No. 4,039,372.

One unforeseen problem which arises when the latter operations are usedis that, to achieve the same final pulp brightness, chemical consumptionin the first bleaching stage is increased when compared with bleachingeffected in the absence of such use of effluents from the bleach plant.The increase in chemical consumption has been found to result from thepresence of dissolved organic material in the aqueous phase of thedilute pulp suspension. The dissolved organic material consumes somebleaching chemicals, thereby, increasing chemical consumption.

The term "dissolved organic material" as used herein refers to bleachingchemical-consuming organic material dissolved in the aqueous phase ofthe pulp suspension and is measured in terms of total organic carbon(TOC).

SUMMARY OF INVENTION

It has now surprisingly been found that improved first stage bleachingmay be effected in the presence of such dissolved organic material byutilizing a serial addition of particular bleaching chemical solutionswithout intermediate washing between the separate bleaching chemicaladdition steps, to result thereby in a decreased chemical consumption ascompared with the application of the bleaching chemicals all at one timein the bleaching stage.

In accordance with the present invention, there is provided a multi-stepprocess, wherein a gaseous mixture of chlorine dioxide, chlorine andsteam is formed using a specific procedure, an aqueous solution ofchlorine dioxide and chlorine having a specific concentration range isformed from the gaseous mixture, and a cellulosic fibrous material pulpis bleached in an aqueous suspension thereof at a specific consistencyand in the presence of a specific concentration of dissolved organicmaterial using the aqueous solution, in a serial application with anaqueous chlorine solution without an intermediate washing step underspecific conditions.

The present invention, therefore, utilizes a novel combination of veryspecific operating conditions to provide more efficient chemical usagein bleaching when dissolved organic material is present in the pulpsuspension.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graphical representation of the effect of various quantitiesof chlorine with chlorine dioxide, expressed as the percentage of totalavailable chlorine in the chlorine dioxide and chlorine solution, in thefirst step of serial bleaching (D/C→C) in comparison with mixturesapplication (D/C); and

FIG. 2 is a flow sheet illustrating one embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION

Referring to FIG. 2 which is a flow sheet of the current best mode ofeffecting the invention known to the applicants, a chlorine dioxidegenerator 10 produces a gaseous product stream 12 containing chlorinedioxide, chlorine and steam.

The chlorine dioxide generator 10 has a unilocular reaction zonecontaining an aqueous acid reaction medium which is maintained at atemperature of about 55° to about 85° C. while a subatmospheric pressureof about 80 to about 300 mm Hg is applied to the reaction zone tomaintain the same at its boiling point. An air bleed in line 14 isprovided.

Reactants are continuously fed to the reaction medium in the form ofsodium chlorate in line 16, sodium chloride in line 18 and sulphuricacid is fed by line 20. Hydrochloric acid may be used to replace all orpart of the sodium chloride, in which case the hydrochloric acid alsoreplaces part of the sulphuric acid. The feed of sulphuric acidmaintains the total acid normality of the reaction medium in the rangeof about 2 to about 4.8 normal. Once saturation of the reaction mediumis reached after start up, anhydrous neutral sodium sulphateprecipitates from the reaction medium and is removed by line 22.

The reactions which occur in the chlorine dioxide generator 10 may berepresented by the equations:

    NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 →ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O+Na.sub.2 SO.sub.4                                       (1)

    NaClO.sub.3 +5NaCl+3H.sub.2 SO.sub.4 →3Cl.sub.2 +3H.sub.2 O+3Na.sub.2 SO.sub.4                                      (2)

The reaction of equation (1) predominates in the process and the extentto which that reaction predominates constitutes the efficiency of thechlorine dioxide producing process.

The production of chlorine dioxide by the latter procedure hasconsiderable benefits over prior chlorine dioxide-producing processesand these have lead to wide commercial practise of the processthroughout the world. The process is known as the ERCO R3 (trademarks)process and is the subject of U.S. Pat. No. 3,864,456, assigned to theassignee of this application.

Although the invention is described with particular reference to the R3process, the invention is applicable to and effective with otherchlorine dioxide and chlorine producing processes wherein added chlorineion is used as the reducing agent for chlorate in the aqueous acidreaction medium while the reaction medium is maintained at its boilingpoint under a subatmospheric pressure, whereby the chlorine dioxide andchlorine is admixed with steam as the diluent gas therefor.

Examples of other chlorine dioxide and chlorine producing processeswhich may be used in place of the procedure of U.S. Pat. No. 3,864,456are the ERCO R5 (trademarks) process, described in Canadian Pat. No.913,328 and U.S. Pat. No. 4,075,308, and the ERCO R6 (trademarks)process, described in U.S. Pat. No. 3,929,974, wherein the sodiumchlorate is reduced by hydrochloric acid, which provides all thechloride ions and all the hydrogen ions required for the process. Thereactions involved are summarized by the following equations:

    NaClO.sub.3 +2HCl→ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O+NaCl (3)

    6NaClO.sub.3 +6HCl→3Cl.sub.2 +3H.sub.2 O+6NaCl      (4)

wherein equations (3) and (4) correspond to equations (1) and (2) forthe R3 process.

Another chlorine dioxide and chlorine producing process which may beutilized in the present invention is the ERCO R7 (trademarks) process,described in U.S. Pat. No. 4,086,329, wherein sulphuric acid andhydrochloric acid used in the reduction reaction are mainly regeneratedinternally by reaction of chlorine with sulphur dioxide and water.

The gaseous product stream in line 12 consisting of chlorine dioxide,chlorine, steam and a small amount of air is conveyed to a condensor 24wherein the gas stream is cooled to a temperature of about 3° to about60° C., preferably about 7° to about 60° C., to cause partialcondensation of the steam therein.

The gas stream in line 26 is fed into countercurrent contact in acontact tower 28 with a stream of chilled water in line 30 having atemperature of about 0° to about 22° C., preferably about 3° to about10° C., to dissolve all the chlorine dioxide from the gaseous streamalong with part of the chlorine.

The aqueous solution of chlorine dioxide and chlorine leaving the tower28 in line 32 usually has a concentration of chlorine dioxide of about 8to about 9 grams per liter and a concentration of chlorine of about 1.5to about 1.8 grams per liter, the actual concentrations being dependenton the temperature of the chilled water in line 30 and the flow rate ofwater to the tower 28. At the same flow rate, the concentrations ofchlorine dioxide and chlorine increase with decreasing temperature, sothat colder rather than warmer temperatures often are preferred.

The concentration of chlorine present in the aqueous solution ofchlorine dioxide and chlorine in line 32 is always about 6.0% and up to10.0% of the total available chlorine of that solution, preferably about7 to 8%.

The term "total available chlorine" is used herein in its normal meaningin the bleaching art and refers to the total bleaching power of thesolution, chlorine dioxide having a bleaching power which is 2.63 timesthat of chlorine on a weight basis.

Solutions of such concentrations contrast markedly with chlorine dioxidesolutions obtained from other chlorine dioxide-producing processes. Inthose cases where no chloride ion is added and substantially purechlorine dioxide is formed, the aqueous solution contains a negligiblechlorine concentration. For those processes where chloride ion is addedas the reducing agent and chlorine is produced along with the chlorinedioxide, such as, in the ERCO R2 (trademarks) process, which is thesubject of U.S. Pat. No. 2,863,722, wherein air is used to dilute thegases, the chlorine dioxide solution produced by absorption in watercontains some dissolved chlorine, but the concentration is well belowthe concentration thereof in the chlorine dioxide and chlorine solutionpresent in line 32. Under typical operating conditions, the differencesin partial pressure of the gases in the processes result in an aqueoussolutiion of chlorine dioxide and chlorine containing 8 grams per literchlorine dioxide and 1.7 grams per liter chlorine in the case of the R3or similar process and an aqueous solution of chlorine dioxide andchlorine containing 8 grams per liter chlorine dioxide and 0.5 grams perliter chlorine in the case of the R2 process.

Such prior "technical chlorine dioxide solutions" may contain up toabout 10% by weight of chlorine, corresponding to 4% on an availablechlorine basis, although the applicants are aware that the literaturerefers to values as high as 5.7% on an available chlorine basis. Thetypical R2 process solution contains about 2.3% on an available chlorinebasis.

The difference in chlorine concentrations between those provided byprior procedures and those resulting in this invention arises from thedifferences in partial pressure of chlorine dioxide and chlorine in thegaseous streams which are contacted by water in the absorption tower.

The gaseous product stream resulting from the contact tower 28 consistsof chlorine and air in line 34. The vacuum in the generator 10 isusually applied via the gaseous stream in line 34, typically afterdissolving the chlorine therefrom in a convenient aqueous medium. Avacuum pump of any desired type may be used to apply the vacuum, and thevalue of the subatmospheric pressure is controlled by the air bleed tothe generator 10 in line 14.

Usually the chlorine dioxide generator 10 is provided with sufficientcapacity that the chlorine dioxide and chlorine solution in line 32 isof sufficient volume for use in all bleaching operations requiring theuse of chlorine dioxide. For convenience of illustration, the chlorinedioxide solution in line 32 is shown as being passed to the first stage36 of a multistage bleaching and purification operation effected on awashed but otherwise untreated wood pulp in line 38, although it will beunderstood that usually only a proportion thereof passes to the firststage of the bleach plant.

The process of the present invention is applicable to any cellulosicfibrous material but is described with particular reference to thebleaching of wood pulp, preferably wood pulp produced by the Kraftprocess, i.e., wood pulp produced by digestion of wood chips in apulping liquor containing sodium hydroxide and sodium sulphide as theactive pulping chemicals.

The term "bleaching stage" as used herein refers to a pulp bleachingoperation effected between other pulp treatments, usually washings. Theterm "bleaching step" as used herein refers to a pulp bleachingtreatment effected within a bleaching stage.

In the first bleaching stage 36, the pulp suspension, of consistency ofabout 2 to about 16% by weight of pulp and containing dissolved organicmaterial in an amount of about 2 to about 10% by weight TOC on pulp, isbleached by a serial application of the bleaching solutions. The pulpsuspension is subjected to a first bleaching step with the chlorinedioxide and chlorine solution fed by line 32 at a temperature of about35° to about 70° C., preferably about 50° to about 65° C. The firstbleaching step may be effected at any desired acid pH value, usually ata pulp suspension pH of about 1 to about 6.

After a period of time from about 5 seconds to about 10 minutes, usuallyabout 30 seconds to about 5 minutes, has lapsed from the application ofthe aqueous solution of chlorine dioxide and chlorine, an aqueouschlorine solution fed by line 40 is applied to the pulp suspension. Thechlorine solution in line 40 may be provided from any convenient source,for example, by absorbing the chlorine gas in line 34 in water. Thechlorine treatment may be effected at any desired acid pH value, usuallyat a pulp suspension pH of about 0.7 to about 3. The bleaching procedureis allowed to proceed for about 10 to about 60 minutes at the bleachingtemperature of about 35° to about 70° C.

The overall amount of available chlorine applied to the pulp in thefirst and second bleaching steps, both as chlorine dioxide and chlorine,usually is about 2 to about 10% by weight of the pulp. The chlorinedioxide in the aqueous solution of chlorine dioxide and chlorineconstitutes about 20 to about 90%, preferably about 30 to about 75%, ofthe total available chlorine used in the first and second bleachingsteps.

Following completion of the bleaching stage, the pulp passes by line 42to the remainder of the multistage bleaching and caustic extractionoperations 44, wherein the pulp is subjected to EDED steps using anaqueous solution of sodium hydroxide fed by line 46 in the E-stages, andan aqueous solution of chlorine dioxide fed by line 48 in the D-stages,usually part of the chlorine dioxide and chlorine solution in line 32.Intermediate washing is usually effected between each chemicalapplication stage, using water fed by line 50. Other bleaching agents,such as, hypochlorite and peroxide may be used in the bleaching steps.The final bleached pulp is removed by line 52.

It is preferred to effect countercurrent flow of aqueous media withrespect to pulp flow through the bleaching and purification operations,as discussed above. This operation results in the presence of about 3.5to about 6.5% by weight TOC on pulp in the pulp suspension which isbleached in the first bleaching stage 36.

The bleaching and purification operations effected in 36 and 44 may becarried out by the so-called "dynamic bleaching" process, as describedin Canadian Pat. No. 783,483, in which case the washing steps usuallyare omitted.

EXAMPLES Example I

This Example illustrates the effect of the serial application of anaqueous solution of chlorine dioxide and chlorine followed by chlorine,when compared with the mixtures application of a solution of chlorinedioxide containing all the chlorine and the serial application of anaqueous solution of chlorine dioxide followed by chlorine.

A series of experiments was carried out on samples of a wood pump havinga Kappa number of 34.2, a K number of 23.4 and a viscosity of 28.8 cpswith associated pulp mill filtrate. Bleaching was effected at 60° C. and3.5% consistency for 30 minutes at an applied chlorine value of 7.6% byweight of pulp, with 70% of the available chlorine being provided bychlorine dioxide and 30% of the available chlorine being provided bychlorine.

In one set of experiments, the chlorine dioxide was applied as anaqueous solution and the chlorine was applied 1 minute after thechlorine dioxide application without an intermediate wash at varying %TOC levels in the filtrate.

In another set of experiments at varying % TOC levels in the filtrate,the chlorine dioxide was applied as an aqueous solution of chlorinedioxide containing part of the chlorine in an amount equivalent to 5% ofthe overall available chlorine, so that the chlorine provided about 7%of the available chlorine of the solution and was equivalent to atypical solution of chlorine dioxide and chlorine obtained from the ERCOR3 process, as described above. One minute following application of thechlorine dioxide and chlorine solution, the remainder of the chlorine(25% of the overall available chlorine) was applied without anintermediate wash.

In a further set of experiments, an aqueous solution of chlorine dioxideand chlorine again was applied, in this case containing chlorine in anamount equivalent to 10% of the overall available chlorine, so that thechlorine provided about 13% of the available chlorine of the chlorinedioxide and chlorine solution. The remainder of the chlorine (20% of theoverall available chlorine) was applied 1 minute after the aqueoussolution application without an intermediate wash.

In a yet further set of experiments, the bleaching was effected with anaqueous solution containing all the chlorine dioxide and all thechlorine.

For each set of experiments, the Kappa number of the pulp was determinedafter the pulp had been washed, alkali extracted using 3% NaOH on pulpfor two hours at 70° C. and 12% consistency, and again washed. The Kappanumber determinations were plotted against % TOC on pulp and theresulting graphical representations appear as FIG. 1. The dotted linelabelled D/C and the dotted line labelled 0% are the trend lines for theresults of the fourth and first sets of experiments respectively, theactual point determinations being omitted for ease of illustration. Thesymbol "→" signifies the serial application of bleaching chemicalswithout an intermediate wash.

As may be seen from FIG. 1, the results obtained at 7% availablechlorine of the chlorine dioxide and chlorine solution provided bychlorine represent a considerable improvement over the results obtainedusing a single mixtures application and are not significantly differentfrom the case where the initially-applied chlorine dioxide solutioncontains no chlorine.

The latter results are important in that they demonstrate that anaqueous chlorine dioxide and chlorine solution produced by the ERCO R3process, may be used in the first step of the serial application toobtain results not significantly different from pure chlorine dioxidesolution, without the necessity for purification by way of chlorineremoval from the solution.

The results obtained at 13% of the available chlorine of the chlorinedioxide solution provided by chlorine represent an improvement over theresults obtained using a single mixtures application but are moresignificantly different from the 0% case than the 7% case.

Example II

This Example illustrates the results attained at another proportion ofClO₂ to Cl₂ in the bleaching stage.

A softwood Kraft pulp, having a 20.8 K number, 30.9 Kappa number and aviscosity of 31.1 cps, was bleached at 60° C. and 3.5% consistency for30 minutes at an applied equivalent Cl₂ value of 6.9% using 60% of theavailable chlorine as ClO₂ and 40% of the available chlorine as Cl₂, inthe presence of dissolved organic material measuring 6.3% TOC on pulp.

After this bleaching step, the pulp was washed, caustic extracted using2.8% NaOH on pulp at 12% consistency for 2 hours at 70° C., and thenwashed again. The pulp then was tested for its properties.

In one test, an aqueous solution of chlorine dioxide and chlorinecontaining all the chlorine dioxide, i.e., 60% of the availablechlorine, and 4% Cl₂, as available chlorine (i.e., a solution in whichthe total available chlorine was provided approximately 93% ClO₂ and 7%Cl₂) was first applied to the pulp, followed 21/2 minutes later withoutan intermediate wash by an aqueous solution of the remainder of thechlorine, i.e., 36% of the available chlorine.

In a second test, the same aqueous solution of chlorine dioxide andchlorine was used in the first application but in the secondapplication, applied 21/2 minutes after the first application without anintermediate wash, there was used an aqueous solution containing 36%available chlorine provided 4% by chlorine dioxide and 32% by chlorine.

In a third test, the chlorine dioxide was first added as an aqueoussolution thereof 21/2 minutes prior to the addition, without anyintermediate washing, of the chlorine as an aqueous solution thereof.

In a fourth test, the chlorine dioxide and chlorine were added as anaqueous solution thereof containing all the ClO₂ and Cl₂.

The results obtained are outlined in the following Table:

                  TABLE                                                           ______________________________________                                        Serial Bleach                                                                 Step 1  Step 2                      Viscosity                                 D/C     D/C     Kappa No.   Kappa No.                                                                             cps.                                      ______________________________________                                        60/4    0/36    5.23        4.05    26.8                                      60/4    4/32    5.75        4.44    26.7                                      60/0    0/40    5.68        4.44    26.3                                       60/40  --      7.73        5.87    26.3                                      ______________________________________                                    

The results of the above Table indicate the improvement in Kappa and Knumbers achieved using the serial application of bleaching chemicals inthe first bleaching stage as compared with mixtures, and theinsignificant difference in pulp properties attained by the use ofchlorine dioxide and limited quantities of chlorine in the first step,when compared with all chlorine dioxide in the first step of the serialchemicals application.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention is directed to ableaching procedure which permits the adverse effects of the presence ofdissolved organic carbon to be overcome and chemical usage economies tobe effected. Modifications are possible within the scope of theinvention.

What we claim is:
 1. A method of bleaching pulp using chlorine dioxide, which comprises:(a) continuously forming a gaseous mixture of chlorine dioxide, chlorine and steam by:(i) continuously feeding a sodium chlorate solution to an aqueous acid reaction medium present in a unilocular reaction zone, (ii) continuously feeding sulphuric acid to said aqueous reaction medium in an amount to maintain the total acid normality of the reaction medium in the range of about 2 to about 4.8 normal, (iii) continuously feeding sodium chloride, hydrochloric acid or a mixture of sodium chloride and hydrochloric acid to said aqueous reaction medium, (iv) continuously maintaining said reaction medium at a temperature of about 55° to about 85° C. while maintaining said reaction zone under a subatmospheric pressure of about 80 to about 300 mm Hg to cause the formation of chlorine dioxide and chlorine and the evaporation of water from the reaction medium, (v) continuously depositing anhydrous neutral sodium sulphate from the reaction medium in said reaction zone once the reaction medium becomes saturated thereby after start up, and (vi) continuously removing the gaseous mixture of chlorine dioxide, chlorine and steam from the reaction zone; (b) continuously cooling said gaseous stream to a temperature of about 3° to about 60° C. to cause condensation of at least a substantial proportion of the steam therefrom to provide a chlorine dioxide- and chlorine-containing gas stream; (c) continuously contacting the latter gas stream with water having a temperature of about 0° to about 22° C. at a flow rate sufficient to form an aqueous solution of chlorine dioxide and chlorine containing chlorine in an amount of 6 to 10% of the available chlorine of the solution, and a gaseous chlorine stream; and (d) bleaching a cellulosic fibrous material pulp for about 10 to about 60 minutes at a temperature of about 35° to about 70° C. in an aqueous suspension having a consistency of about 2 to about 16% by weight of pulp and containing dissolved organic material in an amount of greater than 2 to about 10% by weight TOC on pulp at an overall equivalent chlorine concentration of about 2 to about 10% by weight of the pulp, by:(i) subjecting said suspension to a first bleaching step at an acid pH value using at least part of said aqueous solution of chlorine dioxide and chlorine formed in step (c), and (ii) without an intermediate washing step, subjecting the suspension to a second bleaching step at an acid pH using chlorine solution, formed at least part of said gaseous chlorine stream from step (c), and commencing about 5 seconds to about 10 minutes after commencement of said first bleaching step,the chlorine dioxide in said aqueous solution of chlorine dioxide and chlorine constituting about 20 to about 90% of the total available chlorine used in said first and second bleaching steps.
 2. The method of claim 1 wherein said gaseous stream is continuously cooled to a temperature of about 7° to about 60° C.
 3. The method of claim 1 wherein said water has a temperature of about 3° to about 10° C.
 4. The method of claim 1 wherein said gaseous stream is cooled to a temperature of about 7° to about 60° C. to effect said condensation and said water has a temperature of about 3° to 10° C.
 5. The method of claim 1, 2, 3 or 4 wherein said chlorine dioxide and chlorine solution contains chlorine in an amount of about 7 to 8% of the available chlorine of the solution.
 6. The method of claim 1 wherein said dissolved organic material content of said suspension is about 3.5 to about 6.5% by weight TOC on pulp.
 7. The method of claim 1 wherein said cellulosic fibrous material is wood.
 8. The method of claim 1 wherein said first and second bleaching steps are effected at a temperature of about 50° to about 65° C. and the second bleaching step is commenced about 30 seconds to about 5 minutes after commencement of the first bleaching step.
 9. The method of claim 1 wherein said chlorine dioxide in said aqueous solution of chlorine dioxide and chlorine constitutes about 30 to about 75% of the total available chlorine used in said first and second bleaching steps.
 10. The method of claim 1 wherein said pulp suspension has a pH of about 1 to about 6 in said first bleaching step and a pH of about 0.7 to about 3 in said second bleaching step.
 11. The method of claim 1 wherein:(a) said cellulosic fibrous material is wood; (b) said first and second bleaching steps are effected at a temperature of about 50° to about 65° C.; (c) said second bleaching step is commenced about 30 seconds to about 5 minutes after commencement of the first bleaching step; (d) said first bleaching step is effected at a pulp suspension pH of about 1 to about 6; (e) said second bleaching step is effected at a pulp suspension pH of about 0.7 to about 3; and (f) said chlorine dioxide in said aqueous solution of chlorine dioxide and chlorine constitutes about 30 to about 75% of the total available chlorine used in said first and second bleaching steps.
 12. The method of claim 11 wherein said dissolved organic material content of said suspension is about 3.5 to about 6.5% by weight TOC on pulp.
 13. A method of bleaching pulp using chlorine dioxide, which comprises:(a) continuously forming a gaseous mixture of chlorine dioxide, chlorine and steam by:(i) continuously feeding a sodium chlorate solution to an aqueous acid reaction medium present in a unilocular reaction zone, (ii) continuously feeding sulphuric acid to said aqueous reaction medium in an amount to maintain the total acid normality of the reaction medium in the range of about 2 to about 4.8 normal, (iii) continuously feeding sodium chloride, hydrochloric acid or a mixture of sodium chloride and hydrochloric acid to said aqueous reaction medium, (iv) continuously maintaining said reaction medium at a temperature of about 55° to about 85° C. while maintaining said reaction zone under a subatmospheric pressure of about 80 to about 300 mm Hg to cause the formation of chlorine dioxide and chlorine and the evaporation of water from the reaction medium; (v) continuously depositing anhydrous neutral sodium sulphate from the reaction medium in said reaction medium in said reaction zone once the reaction medium becomes saturated thereby after start up, and (vi) continuously removing the gaseous mixture of chlorine dioxide, chlorine and steam from the reaction zone; (b) continuously cooling said gaseous stream to a temperature of about 3° to about 60° C. to cause condensation of at least a substantial proportion of the steam therefrom to provide a chlorine dioxide- and chlorine-containing gas stream; (c) continuously contacting said latter stream with water having a temperature of about 0° to about 22° C. at a flow rate sufficient to form an aqueous solution of chlorine dioxide and chlorine containing about 8 to about 9 grams per liter of chlorine dioxide and about 1.5 to about 1.8 grams per liter of chlorine, and a gaseous chlorine stream; and (d) bleaching a cellulosic fibrous material pulp for about 10 to about 60 minutes at a temperature of about 35° to about 70° C. in an aqueous suspension having a consistency of about 2 to about 16% by weight of pulp and containing dissolved organic material in an amount of greater than 2 to about 10% by weight TOC on pulp at an overall equivalent chlorine concentration of about 2 to about 10% by weight of the pulp, by:(i) subjecting said suspension to a first bleaching step at an acid pH value using at least part of said aqueous solution of chlorine dioxide and chlorine formed in step (c), and (ii) without an intermediate washing step, subjecting the suspension to a second bleaching step at an acid pH using a chlorine solution, formed from at least part of said gaseous chlorine stream from step (c) and commencing about 5 seconds to about 10 minutes after commencement of said first bleaching step,the chlorine dioxide in said aqueous solution of chlorine dioxide and chlorine constituting about 20 to about 90% of the total available chlorine used in said first and second bleaching steps.
 14. The method of claim 13 wherein said gaseous stream is cooled to a temperature of about 7° to about 60° C. to effect said condensation and said water has a temperature of about 3° to about 10° C. 